MacNN notes Apple’s CEO in Decade of Mac OS upgrades likely (it’s easier to link to MacNN than the original NYT article), commenting that Mac OS X Leopard will form the basis for the next ten years of operating systems upgrade. I remember Mac OS X being announced — not sure if this was at a WWDC or Macworld keynote — as “the next twenty years” of Apple operating systems. We’re basically ten years out from the NeXT acquisition of Apple acquisition of NeXT.
He says "I’m quite pleased with the pace of new operating systems every 12 to 18 months for the foreseeable future," wherein we see again that Apple considers the Intel version of Mac OS X to be a major release of Mac OS X. That’s fine, I’m sure it took a lot of effort and it has definitely had an impact. Otherwise, accounting for the 910 days between the debuts of Leopard and Tiger would mean that Apple isn’t sticking to a 12- to 18-month release schedule.
I think, by this logic, it’s entirely reasonable for Microsoft to consider Windows XP SP2, XP 64-bit, XP Tablet Edition, XP Media Center Edition, and all of Windows Vista to be major releases. It’s only fair. Sure, it took them a long time to release an upgrade to the original Windows XP, but it’s not like they’ve sat idle without releasing anything new at all between 2002 and 2007.